Infectivity of certain viruses, particularly HIV and related viruses, such as SIV, SHIV and other immune deficiency viruses, is mediated by the co-receptor, CCR5. This co-receptor has been identified as a major target for HIV-1 entry inhibitors, since most of the viruses responsible for person to person transmission have been typed as CCR5-using (R5) strains. The naturally occurring Δ32 ccr5 allele, when homozygous, is associated with resistance to in vitro infection of CD4+ cells with R5 viruses. Moreover, Δ32 ccr5 homozygosity confers considerable protection against HIV infection in vivo. Yet this genotype is not associated with abnormal immune function, and may be dispensable due to redundancy in chemokine receptor usage.
Three main classes of CCR5-targeting inhibitors have been reported: CC-chemokine analogues, small molecules, and monoclonal antibodies. One of the most active monoclonal antibodies targeting CCR5 is mAb 2D7, which was generated from the spleen of C57BL/6 mice immunized with the murine pre-B cell lymphoma line L1.2, which expresses high levels of transfected CCR5. This murine antibody was shown to inhibit in vitro infections of CD4+CCR5+ human cells by most R5-tropic viruses at an ID50 of 2-10 μg/ml, making it a good candidate for generating humanized antibodies. However, no success in humanizing this mAb has been reported. The epitope recognized by mAb 2D7 on CCR5 has been partially mapped to the first half of the second extracellular loop (ECL-2) by mutagenesis studies. Amino acids 171-KE-172 were found to be critical for mAb 2D7 binding. But the epitope was determined to be conformation-dependent, and the binding is lost in CCR5 mutants lacking the disulfide bridge between ECL-1 and ECL-2, as well as in reduced forms of CCR5 extracted from cells with various detergents.
It would be desirable to identify the conformational epitope recognised by mAb 2D7. A peptide that closely mimics this epitope could be useful, for example, for identifying a human monoclonal antibody against human CCR5. The peptide could also be useful for developing vaccines or other infection-blocking HIV-1 therapeutics.